Project no 10

The insect mustard oil bomb: a chemical weapon against predators and pathogens?

Supervisors: Dr. Franziska Beran Research Group Detoxification and Sequestration in Insects, Max Planck Institute for Chemical Ecology Dr. Hannah Rowland Research Group Predators and Prey, Max Planck Institute for Chemical Ecology

Background: Plants and animals are well-known to use chemical weapons to defend themselves against enemies. When this chemical defence is produced by the organism itself, a major challenge is to store the toxin safely, but ready for action when needed. Plants have solved this problem by developing two-component defence systems (1). The best studied example is the so-called 'mustard- oil bomb' in crucifers, where high amounts of non-toxic mustard-oil glucosides (glucosinolates) are stored in all plant tissues but separately from a specific β-thioglucosidase enzyme known as myrosinase (2). When the plant tissue is damaged by herbivore feeding, mustard oil glucosides are rapidly degraded to toxic mustard oils (isothiocyanates). These are responsible for the sharp taste of mustard and wasabi. Due to their high reactivity, mustard oils have broad activity against bacteria, fungi, nematodes, and small herbivores. We have previously shown that Phyllotreta flea beetles emit toxic mustard-oils, and that this emission is independent from mustard-oil glucosides in the current food plant (3). We discovered that Phyllotreta beetles accumulate and store mustard-oil glucosides in their body up to a level of 2% of their body weight. The beetles also produce their own myrosinase enzyme (4). Furthermore, recent results revealed that all life stages of Phyllotreta contain mustard oil glucosides, and that high myrosinase activity is present in both larvae and adults. These results strongly suggest a defensive role of the mustard oil bomb for the beetles, but the costs and benefits of this strategy are unknown.

Project description: The goal of the project is to elucidate the roles of sequestered mustard-oil glucosides and mustard oils in Phyllotreta flea beetles in defense against entomopathogenic bacteria, fungi, and nematodes, as well as arthropod and bird predators. Therefore, you will analyze how the insect mustard-oil bomb is organized and controlled to prevent self-intoxication in larvae and adults. To assess the ecological function, you will manipulate the levels of mustard-oil glucosides and myrosinase activity in Phyllotreta by RNA interference and CRISPR-Cas9, and compare the performance and fitness of insects with functional and suppressed chemical defense. You will perform assays with different guilds of beetle ‘enemies’ to assay the effectiveness of their defensive phenotype in deterring predatory attack, and inducing predator avoidance learning, including their colour, behaviour, and chemical defence. Candidate profile: We are searching for a highly motivated student with a scientific and curiosity- driven attitude and a strong interest in interdisciplinary research combining molecular biology, chemistry, and ecology. Excellent communication skills and proficiency in written and spoken English is required.

References

1. A. V. Morant et al., beta-glucosidases as detonators of plant chemical defense. Phytochemistry 69, 1795 (Jun, 2008). 2. B. A. Halkier, J. Gershenzon, Biology and biochemistry of glucosinolates. Annu. Rev. Plant Biol. 57, 303 (2006). 3. F. Beran et al., Male Phyllotreta striolata (F.) produce an aggregation pheromone: identification of male-specific compounds and interaction with host plant volatiles. J. Chem. Ecol. 37, (2011). 4. F. Beran et al., Phyllotreta striolata flea beetles use host plant defense compounds to create their own glucosinolate-myrosinase system. P Natl Acad Sci USA 111, 7349 (May 20, 2014).